1. Field of the Invention
The present invention relates to antenna systems. More specifically, the present invention relates to the use of photonic bandgap crystals as efficient reflectors for broadband antenna systems.
2. Description of the Related Art
Antennas are widely utilized in microwave and millimeter-wave integrated circuits for radiating signals from an integrated chip into free space. These antennas are typically fabricated monolithically on III-V semiconductor substrate materials such as GaAs or InP.
To understand the problems associated with antennas fabricated on semiconductor substrates, one needs to look at the fundamental electromagnetic properties of a conductor on a dielectric surface. Antennas, in general, emit radiation over a well defined three-dimensional angular pattern. For an antenna fabricated on a dielectric substrate with a dielectric constant .epsilon..sub.r, the ratio of the power radiated into the substrate to the power radiated into the air is .epsilon..sub.r.sup.3/2. Thus, a planar antenna on a GaAs substrate (.epsilon..sub.r =12.8) radiates 46 times more power into the substrate than into the air.
Another problem is that the power radiated into the substrate at angles greater than EQU .theta..sub.c =sin.sup.-1 .epsilon..sub.r.sup.-1/2
is totally internally reflected at the top and bottom substrate-air interfaces. In GaAs, for instance, this occurs at an angle of 16 degrees. As a result, the vast majority of the radiated power is trapped in the substrate.
Some of this lost power can be recovered by placing a groundplane (a conducting plane beneath the dielectric) one-quarter wavelength behind the radiating surface of the antenna. This technique is acceptable provided the antenna emits monochromatic radiation. In the case of an antenna that emits a range of frequencies (a broadband antenna), the use of a groundplane will not be effective unless the dielectric constant (.epsilon..sub.r) has a 1/(frequency).sup.2 functional dependence and low loss. No material has been found that exhibits both the low loss and the required .epsilon..sub.r dependence over the large bandwidth that is desired for some antenna systems.
One way to overcome these problems is to use a three-dimensional photonic bandgap crystal as the antenna substrate. A photonic bandgap crystal is a periodic dielectric structure that exhibits a forbidden band of frequencies, or bandgap, in its electromagnetic dispersion relation. These photonic bandgap materials are well known in the art. For example, see K. M. Ho, C. T. Chan and C. M. Soukoulis, "Existence of Photonic Band Gap in Periodic Dielectric Structures", Phys. Rev. Lett. 67, 3152 (1990) and E. Yablonovitch, "Photonic Bandgap Structures", J. Opt. Soc. Am. B 10, 283 (1993).
The effect of a properly designed photonic bandgap crystal substrate on a radiating antenna is to eject all of the radiation from the substrate into free space rather than absorbing the radiation, as is the case with a normal dielectric substrate. The radiation is ejected or expelled from the crystal through Bragg scattering. This concept has been demonstrated and described in E. R. Brown, C. D. Parker and E. Yablonovitch, "Radiation Properties of a Planar Antenna on a Photonic-Crystal Substrate", J. Opt. Soc. Am. B 10, 404 (1993).
This reference describes the design, fabrication and experimental verification of a planar antenna that utilizes a photonic bandgap crystal with a bandgap between 13 and 16 GHz. Although this is an improvement over the conventional dielectric substrates described above, there is still a need for a substrate that will cover a wider range of frequencies (a substrate with a larger bandgap) for broadband planar antenna systems and other applications that require broadband frequency selective surfaces. Currently, one cannot fabricate a single photonic bandgap crystal that will cover a wide range of frequencies.